Disc drives of the type known as "Winchester" disc drives, or hard disc drives, are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator bearing housing opposite to the coil, the actuator bearing housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator bearing housing rotates, the heads are moved radially across the data tracks along an arcuate path.
Disc drives of the current generation are included in desk-top computer systems for office and home environments, as well as in laptop computers which are used wherever their users happen to take them. Because of this wide range of operating environments, the computer systems, as well as the disc drives incorporated in them, must be capable of reliable operation over a wide range of ambient temperatures.
Furthermore, laptop computers in particular can be expected to be subjected to large amounts of mechanical shock as they are moved about. It is common in the industry, therefore, that disc drives be specified to operate over ambient temperature ranges of from, for instance, -5.degree. C. to 60.degree. C., and further be specified to be capable of withstanding mechanical shocks of 200 G or greater without becoming inoperable.
One of the areas of disc drive design which is of particular concern when considering ambient temperature variations and mechanical shock resistance is the system used to mount the discs to the spindle motor. During manufacture, the discs are mounted to the spindle motor in a temperature- and cleanliness-controlled environment. Once mechanical assembly of the disc drive is completed, special servo-writers are used to prerecord servo information on the discs. This servo information is used during operation of the disc drive to control the positioning of the actuator used to move the read/write heads to the desired data location in a manner well known in the industry. Once the servo information has been recorded on the discs, it is assumed by the servo logic that the servo information, and all data subsequently recorded, are on circular tracks that are concentric with relation to the spin axis of the spindle motor. The discs, therefore, must be mounted to the spindle motor in a manner that provides sufficient clamping force to prevent shifting of the discs relative to the spindle motor due to differential thermal expansion of the discs and motor components over the specified temperature range, or due to mechanical shock applied to the host computer system.
Several systems for clamping of the discs to the spindle motor have been described in U.S. Patents, including U.S. Pat. No. 5,528,434, issued Jun. 18, 1996, U.S. Pat. No. 5,517,376, issued May 14, 1996, U.S. Pat. No. 5,452,157, issued Sep. 19, 1995, U.S. Pat. No. 5,333,080, issued Jul. 26, 1994, U.S. Pat. No. 5,274,517, issued Dec. 28, 1993 and U.S. Pat. No. 5,295,030, issued Mar. 15, 1994, all assigned to the assignee of the present invention and all incorporated herein by reference. In each of these incorporated disc clamping systems, the spindle motor of the disc drive includes a disc mounting flange extending radially from the lower end of the spindle motor hub. A first disc is placed over the hub during assembly and brought to rest on this disc mounting flange. An arrangement of disc spacers and additional discs are then alternately placed over the spindle motor hub until the intended "disc stack" is formed. Finally, some type of disc clamp is attached to the spindle motor hub which exerts an axial clamping force against the uppermost disc in the disc stack. This axial clamping force is passed through the discs and disc spacers and squeezes the disc stack between the disc clamp and the disc mounting flange on the spindle motor hub.
From the above description, it would appear that the only element that would need to be considered when designing a disc clamping system would be the disc clamp, with any requirement for additional clamping force being met by an increase in the strength of the disc clamp. However, with the industry trend of size reduction in the overall disc drive, the size of various components within the disc drive has also been reduced, including the thickness of the discs. As the discs have grown thinner, the amount of clamping force that can be applied to the discs without causing mechanical distortion of the discs has also fallen. That is, due to inescapable tolerance variation in the flatness of the disc mounting flange on the spindle motor, the discs themselves and the disc spacers between adjacent discs, as well as the yield strength of the disc material, only a finite amount of axial clamping force can be applied to the inner diameters of the discs before the desired flatness of the disc surfaces is lost.
One type of disc clamp which is used extensively in the industry is the so-called "spring-type" clamp. A spring-type clamp is typically formed of sheet spring material stamp-formed to provide both mounting and force-application features, and commonly consists of three major portions: 1) a central mounting portion; 2) a spring portion extending radially outward from the central mounting portion and; 3) a contact portion adjacent the outer diameter of the spring-type clamp.
While spring-type disc clamps have been seen which employ a single, centrally located mounting screw, it is much more common to utilize a plurality of screws evenly spaced about a diameter just inside the outer diameter of the web portion of the disc clamp. The use of multiple mounting screws placed close to the spring portion provides greater overall clamping force than a single central mounting screw, given the same configuration of the remainder of the disc clamp.
The central mounting portion, also sometimes referred to as a web, typically includes one or more screw holes through which machine screws are inserted into corresponding tapped holes in the upper surface of the spindle motor hub. It is also typical for the web to include an arrangement of tooling holes, aligned with corresponding tooling holes in the upper surface of the spindle motor hub, which are engaged by an assembly tool to maintain the relative position of the spindle motor and disc clamp while the screws used to mount the disc clamp are tightened.
The radially extending spring portion is commonly formed at an angle to the plane of the central mounting portion of the disc clamp, and acts, when the web is displaced into contact with the top of the spindle motor hub, similarly to a "belleville" spring to determine the amount of clamping force applied to the top surface of the uppermost disc in the disc stack.
The contact portion of a typical spring-type disc clamp is a circumferentially formed corrugation at the outermost extent of the spring portion. The corrugation is first formed downward, toward the disc surface, and then back upward, thus forming a contact portion which is substantially circular in section at a fixed diameter from the spin axis of the disc stack, and producing a perimeter wall at the outer extreme of the disc clamp.
One problem encountered in the design of spring-type disc clamps relates to the inescapable build up of dimensional tolerances in the components that make up the disc stack. The combined thickness of the discs and disc spacers in the disc stack has a nominal value which, in combination with the nominal dimension from the top surface of the disc mounting flange on the hub of the spindle motor to the top surface of the hub of the spindle motor, defines the nominal relationship between the top surface of the uppermost disc in the disc stack and the top surface of the hub of the spindle motor. This relationship also determines the amount of displacement that the spring-type disc clamp undergoes during assembly, and thus also determines the amount of clamping force exerted on the disc stack.
Disc clamps of the prior art typically operate with a single spring rate, and the tolerance buildup variation noted above leads to a comparable variation in the amount of clamping force exerted across a number of disc drive units, with disc stacks made up of larger numbers of discs and disc spacers having a potentially larger amount of dimensional tolerance buildup extremes. If the desired amount of clamping force is established by the nominal relationship between the top surface of the uppermost disc in the disc stack and the top surface of the spindle motor hub, then units exhibiting extremes of dimensional tolerance buildup will provide either excessive clamping force--potentially leading to distortion of the uppermost disc--or provide severely reduced clamping force. If the clamping force is too low, then the disc drive may not meet specified limits of mechanical shock or thermal variation, and may allow the discs to be shifted from their intended relationship with the spindle motor hub.
Changing the single spring rate of prior art disc clamps from the nominal value serves only to exacerbate the above-described problem at one or the other extreme of dimensional tolerance variation.
A second problem with disc clamping systems using spring-type disc clamps having a single spring rate lies in their reaction to the application of mechanical shocks in parallel with the spin axis of the spindle motor, commonly referred to as "Z-axis shocks". When Z-axis shocks are applied to the disc drive, the mass of the disc stack components acts inertially against the spring force of the disc clamp. If the relatively low spring rate of typical prior art disc clamps (selected, as noted above, to compensate for variations in the dimensions of the disc stack components) acts in resonance with the mass of the disc stack components, axial unloading of the disc stack can occur. To counter this tendency, it is desirable to provide a disc clamp having a high spring rate.
Prior art spring-type disc clamps are not capable of providing a low spring rate to compensate for disc stack dimensional tolerance build up and a high spring rate to increase tolerance to Z-axis applied mechanical shocks.
The need clearly exists, therefore, for a disc clamping system for a disc drive that compensates for dimensional tolerance buildup in the components of the disc stack, and thus exerts a uniform nominal clamping force across units having disc stacks with a wide variation in dimensional ranges, and that also provides a high tolerance of Z-axis shocks.